I'd likely get a PCI card because it seems easier to set up: Check for which kinds of chipsets you can find Windows 9x drivers for online, try to find a card with one of them on it, install it.

For example, Realtek still appears to have 9x drivers for their RTL8029 (10 Mbit only), RTL8139 (10/100 Mbit) and similar chipsets on their site, and I believe 3Com Etherlink cards also worked well under 9x, although I'm not sure about where to get drivers for these or if the OS might've come with them already.